Heart testing for coronary heart disease, myocardial ischemia, and other abnormal heart conditions is routinely performed using an electrocardiogram (ECG), which represents electrical potentials reflecting the electrical activity of the heart measured via electrodes placed on the patient's skin. The heart's electrical system controls timing of the heartbeat by sending an electrical signal through the cells of the heart. The heart includes conducting cells for carrying the heart's electrical signal, and muscle cells that contract the chambers of the heart as triggered by the heart's electrical signal. The electrical signal starts in a group of cells at the top of the heart called the sinoatrial (SA) node. The signal then travels down through the heart, conducting cell to conducting cell, triggering first the two atria and then the two ventricles. Simplified, each heartbeat occurs by the SA node sending out an electrical impulse. The impulse travels through the upper heart chambers, called “atria”, electrically depolarizing the atria and causing them to contract. The atrioventricular (AV) node of the heart, located on the interatrial septum close to the tricuspid valve, sends an impulse into the lower chambers of the heart, called “ventricles,” via the His-Purkinje system, causing depolarization and contraction of the ventricles. Following the subsequent repolarization of the ventricles, the SA node sends another signal to the atria to contract, restarting the cycle. This pattern and variations therein indicative of disease are detectable in an ECG, and allow medically trained personnel to draw inferences about the heart's condition. However, not every developing abnormality is immediately visible in an ECG, and, consequently, many patients are misdiagnosed as healthy. Furthermore, although ECGs are nowadays typically recorded and displayed electronically, they often go little beyond the printed ECG traces of the past in the type of information they provide and the intuitiveness and convenience with which such information is presented.